fix minor bug in MxArray(cv::Mat) ctor
fix minor bug in MxArray(cv::Mat) ctor

the case when Mat is empty.

CVML/mexopencv,fengzhyuan/mexopencv,fengzhyuan/mexopencv,nanit/mexopencv,bearpaw/mexopencv,nanit/mexopencv,bearpaw/mexopencv,CVML/mexopencv